File system permissions in Linux are crucial for maintaining security and proper functionality. This article will delve into the basics of file permissions, explaining how to operate them using both symbolic and numeric methods. Additionally, we will cover special permissions such as setuid, setgid, and the sticky bit, and discuss the default umask settings.

Basic File Permissions

Symbolic Method

In Linux, every file and directory has associated permissions that dictate who can read, write, or execute them. These permissions are represented symbolically by characters:

• r: Read
• w: Write
• x: Execute

Each file has three sets of these permissions for three categories of users:

• User (u): The owner of the file
• Group (g): The group that owns the file
• Others (o): All other users

The command chmod is used to change file permissions. Here are some examples:

# Grant read permission to the user
chmod u+r file.txt

# Remove write permission from the group
chmod g-w file.txt

# Add execute permission for others
chmod o+x file.txt

# Set read and write permissions for the user, and read-only for group and others
chmod u=rw,g=r,o=r file.txt

Numeric Method

Permissions can also be set using octal (numeric) values. Each permission (read, write, execute) has an associated numeric value:

• r: 4
• w: 2
• x: 1

These values are summed to represent different permissions. For example, read and write permissions are represented as 6 (4 + 2). The chmod command can be used with numeric values as well:

# Set read, write, and execute permissions for the user; read and execute for group; and read-only for others
chmod 755 file.txt

# Set read and write permissions for the user, and read-only for group and others
chmod 644 file.txt

Special Permissions

Setuid

The setuid (set user ID) permission allows a user to run an executable with the file owner’s privileges. This is often used for programs that need to perform tasks requiring higher privileges.

# Set setuid on an executable file
chmod u+s executable

Setgid

The setgid (set group ID) permission, when set on a directory, ensures that files created within the directory inherit the group ownership of the directory.

# Set setgid on a directory
chmod g+s directory

Sticky Bit

The sticky bit, when set on a directory, restricts file deletion within the directory to the file owner or the directory owner.

# Set sticky bit on a directory
chmod +t directory

Default File Permissions with umask

The umask command sets default permissions for new files and directories. It specifies which permissions should be removed when files or directories are created. The umask value is subtracted from the default permissions (666 for files and 777 for directories).

Viewing and Setting umask

# View the current umask value
umask

# Set a new umask value
umask 022

For example, a umask of 022 means that new files will have permissions 644 (666 – 022) and new directories will have permissions 755 (777 – 022).

Conclusion

Understanding and managing file permissions in Linux is essential for maintaining system security and proper access control. Whether using symbolic or numeric methods, it is important to know how to set and modify permissions effectively. Special permissions like setuid, setgid, and the sticky bit add an additional layer of control, while the umask command helps configure default permissions for new files and directories. Mastering these concepts will ensure that you can manage file access efficiently and securely on your Linux system.